Removal of Redwood & Cedar Wood Discolorations

by Jess Stryker, Landscape Architect (CA#2743)

Now that your brand new redwood or cedar fence/deck is installed, as you stand admiring it, you realize it has black blotches on the wood. Some of the black marks appear to be made by a circular saw, others are small spots like paint drops. In still other areas, you may have splotches that look like hazy black clouds, or even black lines. What are they? How did they get there? Most of all, what can you do to get rid of them?

Discoloration on a new redwood fence.

Same fence as above after cleaning and brightening.

Normal Color Variations:

Some color variation in the wood is normal, especially for Redwood. Fresh cut cedar has a cream color. Fresh cut redwood varies in color from cream to a reddish pink. The cream colored wood is sapwood, from the outer perimeter of the tree. The reddish-pink wood is heart wood that is taken from the center of the tree, the red color is due to a chemical called tannin. Tannin is very rot and insect resistant. The tannin is what makes redwood so durable. So the red colored wood will hold up better over time. (That's something to consider when picking out redwood boards at the lumber yard.) Both cedar and redwood age to a silver gray color over time.

New redwood fence with normal color variations.

Weathered redwood fence.

Your first step is to ask yourself "is there really a problem, or is the color variation normal?" Keep in mind that if you allow the wood to age to its natural silver-gray color, most color imperfections on fresh cut cedar or redwood will fade and become much less obvious over the course of 2-3 years. If you dislike the silver-gray color of an older fence/deck, you can easily restore it to a lighter color using an acid spray. However this will not restore the original shades of red & cream, which are lost as the wood ages and the tannins leach and blend in the wood.

Chances are more than one problem is causing the discolorations on your fence/deck. In this article we'll look at how to correct the following common sources of discoloration:

The easiest approach to the problem is to go through each of the steps below in order. They are ordered here starting with the easiest and quickest, and moving along to the more laborious and time consuming. Hopefully the discolorations will be reduced to an acceptable level before you get to the last, most labor-intensive step!

Sponsored links:

Sap and Oil Discoloration:

Start by examining the black colorations. If the spots have a somewhat shiny surface and/or are raised slightly above the wood service they are probably caused by sap that has oozed to the wood surface. Sap will often be sticky if you touch it. Oil spots are less common, but periodically occur when a bit of oil drips on the wood while it is being cut at the sawmill. Sap and oil spots can both be removed using paint thinner. Take a small rag or paper towel, wet it with paint thinner, and then scrub the spot with it. It will take a little vigorous rubbing, but if it is sap or oil, the spot will dissolve and come off. Remember that paint thinner is very flammable, be sure you read the warnings on the can, and dispose of the used paint-thinner soaked rag properly!

Mold, Mildew, & Tannin Stains:

The next step is to go after black marks that are caused by tannin stains, mold or mildew. These are the most common causes of black stains you'll find on redwood and cedar fence/decks. As mentioned before, redwood and cedar wood both contain tannin. Tannin reacts with a number of things, such as steel in tools or nails, to create discoloration. Tannin discoloration due to the saw blade used to cut the wood is very common and often appears as black swirl marks on the wood. Steel nails often cause a black spot around the head of the nail. (To minimize nail spots the nails used in redwood and cedar wood should be stainless steel, aluminum, or double-dipped galvanized.) Steel straps used to hold bundles of wood together for shipping also leave black marks on the wood. You will notice that the black tannin stains are worse on the redder areas of the wood, this is because the redder areas have more tannin in the wood.

Mold and mildew simply grows on the cut boards while they are being shipped or stored. If you want to tell if a discoloration is tannin stains or mold/mildew simply put a drop of bleach on it. If it is mold/mildew it will fade away. Do this in an inconspicuous place. Concentrated bleach will discolor the wood. Some experts recommend a solution of one cup bleach and one cup of TSP in one gallon of water, sprayed on the mold to remove it. (Real TSP, not a TSP substitute.) This diluted bleach/TSP solution does not discolor the wood; however I have found that it is not very effective at removing the mold or mildew.

Oxalic Acid treatment:

Fortunately, you can use a chemical called oxalic acid to remove both tannin stains and mold/mildew. Oxalic acid crystals are sold online and at some hardware stores. It can also be used to remove the gray color from aged wood, it turns the wood to a much lighter yellowish-gray color. It will not restore the old wood to a completely fresh-cut look. Oxalic acid will not strip off previously applied sealers, stains or paint. For those situations you need to use a product made specifically for paint or stain removal.

The lighter board on this old redwood gate has been sprayed with oxalic acid.

Cover any metal parts (except nails) of the fence/deck with plastic to protect them from the acid. Wear gloves and eye protection, remember you are working with acid!

Make a spray solution per the directions on the box. Most people use 4 oz. of oxalic acid crystals per gallon of warm water to make the spray solution. One gallon of solution will typically treat about 400 square feet of wood surface, or about 66 linear feet of a standard 6 foot tall fence.

Wet down the fence/deck with a hose, then allow a few minutes for the excess water to drain off. If water puddles on the wood, sweep it off, you want the wood to be just slightly wet.

Use a standard garden sprayer to spray the oxalic acid solution onto the wet wood. Make sure you use a plastic sprayer that does not have any metal parts. Apply the solution in small sections. If treating a fence, an 8 foot long section is a good area to work with at one time.

Spraying oxalic acid on a fence.

After spraying the solution on the fence/deck spread it evenly and rub it into the wood using a broom or brush. A standard broom works very well. Do not use a wire brush. The black discoloration should start to fade as you rub the acid over it with the broom. Stains that are more persistent may need a second application, and a little scrubbing.

Spreading the oxalic acid solution using a broom.

Spot treat areas that resist fading by spraying with the oxalic acid again, and using a small hand held nylon scrub brush (wear gloves!)

Spot treat and scrub areas that need it.

About 30 minutes after the initial acid application, rinse off the fence/deck with a strong stream of water from a hose. Allow to dry and the black coloration should be gone. The entire wood surface will also be slightly lighter and brighter in color than it was previously.

Rinse with strong stream of water from a hose.

Some experts like to clean the fence/deck with TSP before applying the oxalic acid. This might be especially helpful if there's lots of mold on the wood. Use 1 cup of TSP per gallon of warm water and apply similar to the oxalic acid described above. Be sure to use real TSP not one of the TSP substitutes. Some TSP substitutes will turn your fence/deck black.

Other Wood Brightener and Restoration Products:

Many hardware stores no longer stock oxalic acid, making it difficult to find. If you have trouble finding it, try online, or try looking in a local home improvement store for a cedar and redwood cleaner/brightener product that contains oxalic acid. These products are normally found on the same aisle as wood stains. Many of these products are a true one step solution containing both a cleaner, and oxalic acid. Check the ingredients list on the label to see if it contains oxalic acid. Many wood cleaning products don't have oxalic acid in them, while they work good on pine or other common woods the oxalic acid is critical for cleaning cedar or redwood. The product I used in the photos with this article is "Olympic Cedar and Redwood Deck Brightener". The primary ingredient in it is oxalic acid, with a surfactant mixed in to make it penetrate and spread better on the wood. Follow the instructions for product application on the product's label.

Olympic Cedar and Redwood Deck Brightener.

Burns and other causes of discoloration:

Sometimes black marks on redwood and cedar are the result of burning of the wood by a saw blade. Burning typically occurs when the wood jams in the saw and friction from the saw heats the wood and burns it. To remove burn marks sand them off using medium or coarse sandpaper. Standing is the final solution for removing any discolorations that the above methods won't remove.